In most prior art digital computers, as many as three display registers are provided to aid the computer user in debugging or finding errors in a program stored in the computer when the user single-steps through the program by depressing or actuating a single-step key of the computer. Each time the single-step key is actuated, a single program step or instruction of the program is executed, and at least three types of program information are displayed, as indicated below:
(1) the result of the execution is displayed in a first display register;
(2) the next instruction to be executed is displayed in a second display register; and
(3) the step or sequence number of the next instruction to be executed is displayed in a third display register.
Thus, to execute a single program instruction and to display the above three types of program information by means of a single actuation of a single-step key, three display registers are generally required in a digital computer. Such multi-register displays are unduly costly when compared to single-register displays that are utilized in some electronic calculators.
Many known electronic calculators have only a single display, but they often require a multi-step rather than a single-step operation to be performed in order to single-step through a program stored in the calculator. For example, a typical calculator would require approximately four steps to be performed in order for the calculator to display a keycode that is to be executed, to execute said keycode, and to display the results of such execution. Typically, as a first step, a PROGRAM-RUN switch of the calculator would be set to "PROGRAM" representing the program-entry or program display mode of operation of the calculator. As a second step, a SINGLE-STEP key of the calculator would be actuated to display the keycode that is scheduled for execution following the next keycode to be executed. As a third step, the PROGRAM-RUN switch would be reset to "RUN" representing the execution mode of operation of the calculator. As a fourth step, the SINGLE-STEP key would be actuated again, this time to cause the execution of the next keycode to be executed and a display of the results of said execution. An electronic calculator of this type is described in the literature (see, for example, Hewlett-Packard Journal, Vol. 20, No. 1, September 1968). Compared to the multi-register displays of a computer, therefore, an electronic calculator's single display has the advantage of a lower cost display, but the calculator's multi-key or multiple key actuation operation required to step through a program is an operational disadvantage because it increases the chances of operator error as compared with a single-key, single actuation operation.